The present invention relates to a control method of a power steering system for a vehicle having two steered axles. The invention further relates to a power steering system and to a vehicle having two steered axles and equipped with such a power steering system, especially an industrial vehicle such as a heavy truck.
A power steering system has been a standard feature in vehicles for many years, in particular in industrial vehicles. Such a system assists the vehicle driver by applying a force to the wheels, through a steering actuator, which complements the effort, of the driver which is mechanically transmitted from the steeling wheel to the steered wheels of the vehicle. Therefore, a lower effort is required for the driver when turning the steering wheel and the vehicle is easier to manoeuvre.
Also know is a power steering system for a vehicle having two steered axles. For example, the vehicle has two front axles, and the two steered axles are the two front axles. The vehicle then comprises a first steered axle joining two first wheels and a second steered axle joining two second wheels, said second steered axle being located rearward from the first steered axle. Nevertheless, in some cases, one steered axle may be a front axle and the second steered axle may be a rear axle. In these last cases and when the vehicle is turning, the wheels of the rear steered axle are turned in the opposite direction compared to the wheels of the front steered axle.
Typically this power steering system to which the movement of a steering wheel is transmitted via a steering column. The steering gear can comprise a hydraulic actuator. In this case, the movement is assisted by hydraulic pressure from a pump, and then transmitted by a mechanical arrangement to one of the first wheels. A transverse track rod transmits the movement of said first wheel to the other first wheel.
In a first type of conventional system, steering to the second axle can be achieved via one or more link rods fixed on the vehicle frame that mechanically links the steering system of the second axle to the steering system of the first axle, and a second actuator which is hydraulically connected to the hydraulic actuator of the steering gear. The purpose of the link rods is to set the steering angle of the second axle according to the steering angle of the first axle, while the second actuator is used to transmit servo power on the second axle.
However, two significant drawbacks of this first type of conventional power steering system are that it comprises heavy and expensive parts, and that it requires space—while the corresponding allotted space is generally limited. Therefore, the implementation of such a system leads to heavy packaging work and involves some limitations due to the size of the wheels and/or of the frame.
In order to provide a power steering system which requires less space and which has a reduced weight, a second type of conventional system has been proposed. In such a system, steering to the second axle can be achieved via one or several actuators that are not mechanically linked to the first axle, but that are controlled as a function of the steering angle of the first wheels. In practice, there can be provided a second actuator controlled by a controller as a function of a parameter related to the steering wheel movement, and designed to control the steering of at least one of the second wheels, according to a second steering angle which is set according to a rule depending on the first steering angle of the first wheels.
One problem with this second type of conventional system is that, in some conditions, steering the vehicle may be very difficult insofar as too high an effort is required for the driver, because the power steering system does not properly work. For example, this can be due to the fact that either the device providing a steering assistance to the first wheels does not operate normally or sufficiently, or the fact that the operation of the device providing a steering assistance to the second wheels has not been triggered.
As a consequence, this second type of conventional system may not fully comply with the regulations.
It therefore appears that, from several standpoints, there is room for improvement in power steering systems for vehicles.
It is desirable to provide an improved power steering system, for a vehicle having two steered axles, which can overcome the drawbacks of the prior art.
It is also desirable to provide a power steering system which requires less space than the prior art systems and which has a reduced weight, while also being effective and reliable, in that it ensures the provision of a sufficient steering assistance effort in substantially any situation.
According to a first aspect, the invention relates to a control method of a power steering system for a vehicle having two steered axles, i.e. a first steered axle joining two first wheels, and a second steered axle joining two second wheels, the power steering system comprising:                a first steering device designed to control the steering of at least one of the first wheels, according to a first steering angle α1, said first steering device comprising:                    a mechanical transmission of the movement from a steering wheel of the vehicle to at least one of the first wheels;            a steering assistance device including a first actuator controlled as a function of a parameter related to the steering wheel movement and capable of providing a steering assistance effort to at least one of the first wheels;                        a second steering device comprising a second actuator controlled by a controller as a function of a parameter related to the steering wheel movement, and designed to control the steering of at least one of the second wheels, according to a second steering angle α2 which is set according to a first rule F1(α1) depending on the first steering angle α1, in normal operation of the power steering system;        
wherein the control method comprises:                detecting or estimating whether a steering wheel torque is greater than a predetermined threshold;        in case the steering wheel torque is greater than said threshold, operating the second steering device in a forced mode by setting the second steering angle α2 according to a rule which depends on the first steering angle α1 and which takes into account at least one different or additional data, in order to cause an offset-steering of said second wheel.        
In practice, during normal operation, no excessive demands are made on the driver's strength to manoeuvre the vehicle, either solely by the mechanical transmission or through the additional operation of the first and second actuators.
In said normal operation also called “normal mode”, the first steering device is controlled as a function of a parameter related to the steering wheel movement, which can be the steering wheel angle, the steering wheel speed, or the steering effort—or torque—on the steering wheel. The first wheels are thus made to steer according to a first steering angle α1 depending on said parameter.
Moreover, the second wheels are made to steer not through a mechanical link with the first wheels, but by means of the second steering device. More precisely, the second actuator is controlled as a function of a parameter related to the steering wheel movement—for example related to the steering wheel angle—by a controller which, in response to the steering wheel movement, and given the current first steering angle α1, sets the second steering angle α2 according to the following equation: α2=F1(α1). For geometrical and mechanical reasons, α2 should not be identical to α1.
It has to be noted that the first rule F1(α1) can be defined by a function that is linear or not and can further depend on other parameters than α1 such as the vehicle speed, etc.
Simultaneously, detecting means are provided to detect or estimate if the steering wheel torque is greater than a predetermined threshold. Typically, the detecting means can be included in monitoring means for monitoring the steering wheel torque.
In case the steering wheel torque is greater than said threshold, the second steering device is operated in the forced mode, where α2 is determined according to a rule that takes into account at least one different or additional data in such a manner that:                for a given value of α1 and when the torque is applied by the driver on the steering wheel in order to turn the second wheels so that they move away from their neutral position, the absolute value of α2 is greater in the forced mode than the absolute value of α2 determined in the normal mode, the second wheels are in this first case oversteered with respect to what would happen in normal mode, i.e. with respect to the value set by the controller in normal mode given the parameter related to the steering wheel movement;        for a given value of α1 and when the steering wheel torque is applied in order to turn the second wheels so that they move closer to their neutral position, the absolute value of α2 is lower in the forced mode than the absolute value of α2 determined in the normal mode, the second wheels are in this second case understeered with respect to what would happen in normal mode.        
The term “neutral position” means a position where the wheels of a steered axle are in line with the other wheels and are oriented parallel to the vehicle longitudinal axis that extends from the front to the rear of the vehicle.
The term “oversteer” means that the absolute value of the corresponding angle α2 is greater than it would be in normal mode, bearing in mind that the sign of α2 depends on the direction in which the steering wheel is turned.
The term “understeer” means that the absolute value of the corresponding angle α2 is lower than it would be in normal mode.
The term “offset-steer” refers to an oversteer or an understeer of the wheels of the second steered axle depending if the torque is applied by the driver on the steering wheel in order to turn the second wheels so that the move away from their neutral position (oversteer) or is applied in order to turn the second wheels so that they move closer to their neutral position (understeer).
The oversteering or understeering of the second wheels causes the vehicle to turn according to a given direction and the first wheels to tend to follow this direction. Therefore the resistive effort that opposes to the rotating movement of first wheels and that results from the frictional effort between the first wheels and the road is reduced. Consequently, the driver can then more easily manoeuvre the first steering axle, even only through the mechanical transmission. As a results, owing to the operation of the system in a forced mode, the vehicle is made to turn without excessive effort from the driver.
Owing to the fact that an offset-steering is triggered by a steering wheel torque greater than a predetermined threshold, the invention ensures that the power steering system always provides an assistance to reduce effort required by the driver when turning the steering wheel or when he wants to turn the steering wheel. The steering wheel torque threshold has to be adequately set to trigger the forced mode before too great an effort is required from the driver.
The control method may comprise operating the second steering device in a forced mode in case a failure is detected in the operation of the steering assistance device. This can be a failure of the first hydraulic actuator or of another part of the steering assistance device of the first steering device. In this case, the driver may not succeed to manoeuvre the vehicle only with the mechanical transmission and the second steering device operated in normal mode. Owing to the operation in forced mode involving an offset-steering of the second steered axle, an assistance is provided to reduce effort required by the driver when turning the steering wheel or when he wants to turn the steering wheel.
Alternatively, the second steering device may be operated even if the steering assistance device of the first steering device is fully operational, but in case an additional steering assistance is required to prevent the driver from having to exert too great an effort on the steering wheel. This can for example happen if the vehicle is overloaded or in case of very uneven grounds, the resisting effort which the ground exerts on the first wheels of the vehicle then being fairly high.
According to an embodiment of the invention, when the second steering device is operated in the forced mode, the second steering angle α2 is set according to a second rule F2(α1) depending on the first steering angle α1 and different from the first rule F1(α1). The second rule can be defined by a function that is linear or not and can further depend on other parameters than α1 such as the vehicle speed, etc.
In concrete terms, the rule used to calculate α2 as a function of α1 is not the same in normal mode and in forced mode. The threshold value of the steering wheel torque is the data which makes the controller choose the appropriate rule.
The second rule can be defined by F2(α1)=f(α1)+A with f(α1) being identical or different from the first rule F1(α1) and A being a constant value or a function depending on the steering wheel torque and/or on the time.
Preferably, F2 could be such that F2(α1=0)≠0. In other words, the the second wheels can be, for instance, oversteered as soon as the steering wheel torque is greater than said threshold and even if the driver cannot turn the steering wheel. For example F2=f(α1)+A, with A being a constant different from zero.
A linear function representative of the first rule can be, for instance, defined by F1(α1)=r1×α1 and a linear function representative of the second rule can be defined by F2(α1)=r2×α1+A with r1 and r2 being different ratios of α1/α2, r2 being, greater than r1 and A being a constant different from zero.
Alternatively; it could be envisaged that F2(α1=0)=0 in case it is certain that the driver will always succeed in turning the steering wheel by an angle that could be very small but sufficient to cause, for instance, an oversteering of the second wheels, thereby initiating, the turning movement of the vehicle.
In this alternative, the first rule can be defined, for instance, by
F1(α1)=r1×α1 and the second rule can be defined by F2(α1)=r2×α1 with r1 and r2 being different ratios of α1/α2 and r2 being greater than r1
In the preceding embodiments, the ratio r2 that is different from r1 correspond to the different data of the rule used to calculate α2 in the forced mode.
In another alternative, A can be an increasing function of the steering wheel torque, meaning that A increases when the steering wheel torque increases over said threshold. The advantage of such an implementation is that the steering assistance increases when the steering wheel resistive torque continues to increase over said threshold.
A, as a function of the steering wheel torque, can be substantially constant above a predetermined steering wheel torque value greater than said threshold value.
According to another embodiment of the invention, when the second steering device is operated in the forced mode, the second steering angle α2 is set according to the same first rule F1, that takes into account at least one different or additional data when the steering wheel torque is greater than said threshold.
The first rule can be defined so that F1(α1)=g(α1)+h(X), with h(X) being a function depending on an additional data X and which is equal or approximately equal to zero when the steering wheel torque is below said threshold.
The additional data X can be the steering wheel torque SWT so that F1(α1)=g(α1)+h(SWT), with h(SWT)=0 or h(SWT) approximately equal to zero when the steering wheel torque (SWT) is below said threshold (SWTth).
On the one hand, in normal mode (i.e. when the steering wheel torque is below said threshold): α2=F1(α1)=g(α1)+h(X) with h(X)=0.
On the other hand, in forced mode (when the steering wheel torque is above said threshold): α2=F1(α1)=g(α1)+h(X) with h(X)≠0. In other words, in forced mode, it is taken into account an additional data X so that a corresponding value h(X) is added to the rule of normal mode.
In this implementation, the rule by which α2 is set according, to α1 remains the same whatever the operating mode, which can be easier to implement. It is as if the threshold value of the steering wheel torque would be included in the function F1.
Preferably, h(SWT) can be an increasing function of the steering wheel torque, meaning that h(SWT) increases when the steering wheel torque increases over said threshold. The advantage of such an implementation is that the steering assistance increases when the steering wheel resistive torque continues to increase over said threshold.
h(X) can be substantially constant above said threshold value or above a predetermined steering wheel torque value greater than said threshold value.
It has to be noted that h(X) could further depend on other data than the steering wheel torque.
In particular, h(X) can further depend on time t and on the steering wheel torque SWT, so that h(X)=h(t, SWT), where h(t, SWT) is an increasing function of time t if the steering wheel torque remains greater than said threshold. Owing to this progressivity, the driver can control and modulate the efforts exerted on the steering wheel and better control the vehicle operation.
For example, h(t, SWT) can substantially linearly depend on time until an upper plateau, where h(t, SWT) is maintained constant, when the steering wheel torque is greater than said threshold.
Besides, it can be provided that, after the steering wheel torque has exceeded said threshold, h(t, SWT) decreases until zero, as a function of time, if the steering wheel torque becomes lower than said threshold.
Said threshold can be comprised between about 10 and about 20 daN, for example between about 15 and about 20 daN.
The control method can further comprise warning the vehicle driver in case the second steering device is operated in the forced mode. The driver thus knows the vehicle has to be stopped as soon as possible.
According to a second aspect, the invention relates to a power steering system for a vehicle having two steered axles, i.e. a first steered axle joining two first wheels, and a second steered axle joining two second wheels, the power steering, system comprising:                a first steering device designed to control the steering of at least one of the first wheels, according to a first steering angle (α1), said first steering device comprising:                    a mechanical transmission of the movement from a steering wheel of the vehicle to at least one of the first wheels;            a first actuator controlled as a function of a parameter related to the steering wheel movement and capable of providing a steering assistance effort to at least one of the first wheels;                        a second steering device comprising a second actuator controlled by a controller as a function of a parameter related to the steering wheel movement, and designed to control the steering of at least one of the second wheels, according to a second steering angle α2 which is set according to a first rule F1 depending on the first steering angle α1, in normal operation of the power steering system;        
the power steering system further comprising detecting means which are capable of detecting or estimating a steering wheel torque greater than a predetermined threshold, and which are coupled to said controller, the controller being designed to set the second steering angle α2 according to a rule (F2, F1) which depends on the first steering angle (α1) and which takes into account at least one different or additional data, in a forced mode in which the steering wheel torque is greater than said threshold, in order to cause an offset-steering of said second wheel.
For example, the first and second axles are two front axles. The second steered axle can be located rearward from the first steered axle, although the reverse is also possible.
In an implementation, the first actuator and/or the second actuator can be hydraulic actuator(s).
According, to a third aspect, the invention relates to a vehicle having two front steered axles i.e. a first steered axle joining two first wheels, and a second steered axle joining two second wheels, where the second steered axle can be located rearward from the first steered axle, the vehicle comprising a power steering system as previously described.
These and other features and advantages will become apparent upon reading the following description in view of the drawing attached hereto representing, as non-limiting examples, embodiments of a power steering system and a control method thereof, according to the invention.